<!DOCTYPE html>
<html>
	<head>
		<meta charset="UTF-8">
		<title></title>
	</head>
	<body>
		<script>
			var num =prompt("请输入科目的总数：");
			if (isNaN(num)||num<=0){
				alert("请输入正确的科目数：");
			}
			
		</script>
		
	</body>
</html>
